Note - I just noticed that this game was in the "Adventure Legends" portion of the site. Anyway, here are the derivatives of that game that do not appear here:
SLENDER: THE ARRIVAL
http://www.slenderarrival.com/
This is a yet-to-be-released non-freeware expansion of the game.
SLENDER MAN'S SHADOW
http://www.slendermansshadow.com/
This is a series of freeware "episodes" based upon the "SLENDER: THE EIGHT PAGES" game (not known if based on concept or actual game code).
HAUNT: THE REAL SLENDER GAME
http://www.haunt-slendergame.com/
A very-soon-to-be-released freeware game based upon "SLENDER: THE EIGHT PAGES" game (not known if based on concept or actual game code).